As cellular radio communications systems have become more popular, interest has grown in transmitting data signals from modems over such systems in addition to the more traditional voice signals. Most currently available cellular radio communications systems can accommodate modem data signals, typically by treating modem data signal as a voice signal. Such cellular radio communications systems require that the user (mobile subscriber) carry the complete modem hardware to any mobile location where modem data signals are to be generated or received. In an effort to reduce the amount of equipment that a mobile subscriber must transport, the cellular radio communications system industry has proposed a system wherein some of the modem hardware could be located at other locations in the cellular radio communications system itself, rather than entirely at the mobile location(s).
For instance, enhanced systems described in Telecommunications Industry Association's interim standards IS-130 and IS-135 describe a cellular radio communications system where the logical components of the modem are divided between a modem front end and a modem back end which are physically separated, but connected via a radio link. During operation in such systems, a modem front end is located with the mobile unit. However, the complementary modem back end is not located with the mobile unit, but instead resides elsewhere in the cellular radio communications system. Additionally, multiple modem back ends are grouped together in a pool arrangement at their location. In such a cellular radio communications system, data signals are routed by the intervening parts of the cellular radio communications system to a randomly selected modem back end from the pool. As such, any one given modem front end may be coupled to any one of multiple modem back ends to create an operational modem for a given call-attempt. Each subsequent call-attempt will likely result in the same modem front end being coupled with a different modem back end. Thus, the specific components comprising the operational modem, the complementary modem back end and modem front end pairing, will likely change from call-attempt to call-attempt.
Typical data communications software applications, the computer programs that use modems, may not function properly in cellular radio communications systems governed by IS-130 or IS-135, or similar systems. The problem arises from the variable assignment of modem back ends to a given modem front end from call-attempt to call-attempt. In particular, IS-130 or similar cellular radio communications systems are designed so that modem back ends are reset to a default configuration before being assigned to the next data call-attempt. Thus, any non-default modem configuration information from the previous data call-attempt is lost. In short, these type cellular radio communications systems assume that if non-default configuration of the modem back end is desired, such non-default configuration information will be sent during the set-up of each data call-attempt.
However, many communication software programs were designed using a different set of assumptions. Communications software programs send modem configuration information with the first call-attempt of a communications session. However, when a call-attempt fails (e.g., the destination is "busy") many communications software programs assume that the modem used for the next call-attempt in the same communications session will be in the same configuration state as the modem was left in at the end of the previous call-attempt; the programs therefore do not resend the configuration information unless a change to the configuration state is required. For traditional dedicated modems, like those commonly found internal to or physically attached to personal computers, the assumption of configuration state retention is correct. However, due to the variable assignablity found in cellular radio communications systems operating under IS-130, or any other cellular radio communications system where an single modem front end may be connected via a radio link to more than one modem back end during a multiple call-attempt communications session (collectively, IS-130 systems), the assumptions of communication software programs regarding the configuration status of the modem may not hold. Examples of commonly encountered modem configuration information that would not be retained for the next data call-attempt in IS-130 systems include configurations settings for termination character, line feed character, connection timeout, comma time, automatic disconnect delay, response format, call progress monitor, receive line signal selector, query disconnect timer, cellular results code, data compression reporting, data compression selection, frame check sequence, error control reporting, error control selection, call termination, buffer management, modular reporting, modulation selection, V.18 reporting, and V.18 selection. If these configuration settings are not exactly as the communications program expects, the modem will not function properly. Thus, users of many communication software programs operating in a IS-130 environment may experience difficulty any time their data call is not completed on the first try. In addition, these users may experience difficulty any time they attempt to reach multiple destinations, or the same destination multiple times, within the same communication session if any of these connections require the use of non-default modem configurations.